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Full S-param on high gain device with HP8753ES

Question asked by demerad on Aug 26, 2009
Latest reply on Sep 17, 2009 by Dr_joel
we would like to read full s-parameters in one time (S11, S22 and S21). The DUT is a 2.2GHz, 50dB of gain (power)amplifier with input and output isolator. To run this measurment, source power is set to -50dBm (PORT2 receiver input near 0dBm). Reading S21 is not a problem, but S11 and S22 are higher that reality. From 2.2 to 2.4GHz, there were several 'mountains' and 'valley' from 1.1 to 2.0 of vswr.
It' seems that high level on port 2 is a problem to read S11 and S22. So, we measure SWR and gain in 3 steps:
1 - full connection port1/port2  > read S21
2 - disconnect port2 > read S11
3 - reconnect port2, disconnect port 1 > read S22
With this set-up, all parameters are correct.
It's not very easy.
See also picture joined to see good S22 (<1.18)[PORT1 disconnected from amplifier's input] and false(<1.65)[PORT1 connected to amplifier's input]
Please, if someone have an idea, it would be helpfull.
Best regards